This proposal examines the role that memory plays in visual search. More specifically, the specific aims of our studies include determining under what circumstances memory guides attention and what mechanisms and environmental factors are used in memory-guided search. Our preliminary data suggests that memory guides search when search is forced to be serial and only one item can be examined at a time. In contrast, other research has suggested that when more than one item is examined in a single glance, search becomes memoryless. One way to reconcile these disparate findings is to assume that items examined in a single glance are examined in parallel, and parallel systems are inherently memoryless. Individual fixations or samples occur serially and are guided by memory. When visual span within a glance is reduced to one item, search becomes serial, and hence purely memory-guided. In many situations, search can be accomplished by a series of fixations in which more than one item can examined in each glance. In these situations, search is a mixture of serial and parallel processing, and hence becomes less memory-guided. Our second goal is to determine what mechanisms and environmental factors help guided search. This includes examining the contributions of inhibition of return, scan path planning, the use of landmarks, and the fate of previously examined items. To accomplish these goals, we plan to use to a combination of eye tracking, behavioral measures, and mathematical modeling. These studies are important because they give us a greater understanding of the mechanisms involved in visual search and perception and have the possibility of reconciling the serial/parallel dichotomy.